Rationale:The occurrence of N-heterocyclic carbenes in imidazolium-based ionic liquids has long been discussed, but no spectroscopic evidence has been reported yet due to their transient nature. The insertion of an ionizable acid group into the cation scaffold of an ionic liquid which acts as a charge tag allows for the direct detection of free carbenes by mass spectrometry.Methods: Three different Brønsted acidic ionic liquids were synthesized:The speciation of these compounds was then analysed by electrospray ionization ion-trap mass spectrometry in the negative ion mode.
Results:The C2-H deprotonation of the imidazolium cation leading to the formation of the corresponding carbene is highly affected by the basic properties of the counter-anion. In the case of MAICl and MAI À H ionic liquids, no charged species corresponding to the free N-heterocyclic carbene was detected. On the contrary, in the presence of a sufficiently basic anion, such as acetate of MAIAc ionic liquid, an intense signal related to the free carbenic species was observed without the addition of an external base.
Conclusions:In situ formation of free N-heterocyclic carbenes from Brønsted acidic ionic liquids was demonstrated, highlighting the crucial role of anion basicity in promoting the C2-H proton abstraction from imidazolium cations with a carboxylic side chain.
Thin films derived
by the deposition of charged microdroplets generated
in the ESI source of a mass spectrometer act as highly concentrated
reaction vessels in which the final products of an ion–molecule
reaction can be isolated by their precipitation onto a solid surface
under ambient conditions. In this study, the ESI Z-spray source supplied
to a Q-TOF Ultima mass spectrometer was used to investigate the d-fructose acid-catalyzed reactions by microdroplets deposition
onto a stainless-steel target surface. High conversion ratios of d-fructose into 5-hydroxymethylfuraldehyde (5-HMF), 5-methoxymethylfuraldehyde
(5-MMF), and difructrose anhydrides (DFAs) were obtained with HCl
and KHSO4 as metal-free catalysts by using synthetic conditions
under which the same products in bulk are not formed. Furthermore,
the reaction outcome was found to be highly sensitive to the catalyst
and the solvent employed as well as to the ESI source parameters influencing
the thin film formation from microdroplets deposition onto the solid
surface.
The Maillard reaction kinetics in the confined volume of the thin film produced by ESI microdroplet deposition was studied by mass spectrometry. The almost exclusive formation of the Amadori product from the reaction of D-xylose and D-glucose toward L-glycine and L-lysine was demonstrated. The thin film Maillard reaction occurred at a mild synthetic condition under which the same process in solution was not observed. The comparison of the thin film kinetics with that of the reaction performed in solution showed strong thin film rate acceleration factors.
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